Method for treating the surface of a thin porous film material

ABSTRACT

A method for treating the surface of a thin porous film material of tetrafluoroethylene resin is disclosed, which comprises heating said surface to a temperature higher than the thermal decomposition point of said resin so as to decompose and remove part of said surface. The resulting thin porous film material of tetrafluoroethylene resin has an adhesive surface while retaining the desired pore size, hardness, and degree of penetration of adhesive.

This is a divisional of application Ser. No. 07/951,890 filed Sep. 28,1992, which is a divisional of application Ser. No. 07/515,778 filedApr. 26, 1990, which is a continuation of application Ser. No.06/811,805 filed Dec. 20, 1985, now abandoned.

FIELD OF THE INVENTION

The present invention relates to a method for treating the surface of athin porous film material of tetrafluoroethylene resin by heating saidsurface to a temperature higher than the thermal decomposition point ofthe tetrafluoroethylene resin so as to decompose and remove part of saidsurface.

BACKGROUND OF THE INVENTION

Tetrafluoroethylene resins are highly resistant to heat and chemicals,have high electrical insulating properties, provide high waterrepellency and provide good biocompatibility. Bacause of theseadvantages, porous materials of tetrafluoroethylene resins areextensively used in filters, diaphragms, waterproof but airpermeablematerials, electrical coatings, sealants, medical equipment andartificial organs. While several methods have been proposed astechniques for producing porous materials of tetrafluoroethylene resin.

The basic principles of a method depending on drawing operations aredescribed in Japanese Patent Publication No. 13560/1967. The first stepconsists of mixing a powder of tetrafluoroethylene resin with a liquidlubricant and shaping the mixture into a unfired film, tube, or rod bypaste extrusion, calender rolling, or a combination of these twomethods. The subsequent steps comprises (1) removing the liquidlubricant by evaporation or extraction, (2) drawing the dry shapedproduct to make it porous, and (3) firing the porous product to atemperature higher than the melting point of the tetrafluoroethyleneresin so as to set the porous structure.

A method for producing a porous material of tetrafluoroethylene resindescribed in U.S. Pat. No. 462,615 is as follows. A mixture of apolytetrafluoroethylene dispersion and polymethylmethacrylate is kneadedwith preheated rolls, and then injection molded. The product obtained issubjected to a compression molding under heating, and thereafterpolymethylmethacrylate is extracted with acetone, as a result, a porousmaterial of tetrafluoroethylene resin is obtained.

Another method for producing a porous material of tetrafluoroethyleneresin is described in Japanese Patent Application (OPI) No. 56578/73(the term "OPI" used herein means a "published unexamined Japanesepatent application"). In this method, a polytetrafluoroethylenehydrophilic dspersion is mixed with a fine powdered filler and a poreforming agent, and then molded in a thin film. The product is subjectedto a heat treatment, and thereafter the pore forming agent is removed bysolvent extraction, as a result, a porous material oftetrafluoroethylene resin is obtained.

Among the above methods, the method depending on drawing operation ispreferred because this method is good in pore size uniformity andstrength of the product and productivity. However, in the presentinvention there is no limitation on the methods for producing a porousmaterial of tetrafluoroethylene resin.

The structure of the porous material prepared by this process may varyby some degree, depending upon the draw ratio or other drawingconditions such as temperature and speed, but it basically has a fibrousstructure wherein nodes are interconnected by small filaments, providingpores in areas bounded by the filaments and nodes. Generally, byincreasing the draw ratio, the filaments are made longer and the nodessmaller, with the result that the proportion of pores or the porosity,is increased.

One serious problem with the thin porous material of tetrafluoroethyleneresin prepared by the conventional method is that it has low adhesiveproperties, which is an inherent defect of tetrafluoroethylene resins.

Tetrafluoroethylene resins have such a high resistance to chemicals andsolvents that they are not all reactive with acids, alkalis, or organicsolvents. The only substances that are capable of attackingtetrafluoroethylene resins are molten alkali metals, certain solutionsof alkali metals, and hot fluorine and chlorine trifluoride. Therefore,one method available today for modifying the surface of materials oftetrafluoroethylene resins so that they can be bonded to other resins isto treat them with an ammonia solution of metallic sodium. For example,U.S. Pat. No. 3,632,387 discloses a method for modifying the surface ofmaterials of tetrafluoroethylene resins by using an ether solution ofsodium naphthalate or an ammonia solution of metallic potassium.

Another approach that can be taken consists of drawing a porous materialof tetrafluoroethylene resin at a high draw ratio so as to form poresthat are large enough to provide anchorage effects to enable physicaladhesion.

A porous polytetrafluoroethylene has some adhesive property comparedwith a non-porous one. A low viscosity resin can penetrate the pores,and generate the anchorage effect. In the case of a porouspolytetrafluoroethylene having a high porosity and a large pore diameterdue to a high drawing ratio, a resin or a resin solution can penetratethe pore easily. Thus the anchorage effects are increased and theadhesive property is improved.

However, this method, which requires an increase in the overallporosity, and involves great difficulty in producing a porous materialhaving the desired pore size. Further problems arise from the highflexibility of the product, the thin porous material may deform in thebonding step, or the adhesive used will penetrate into the material toan undesirably deep portion. For example, a plurality oftetrafluoroethylene tubes for filter use that have been renderedsufficiently porous to provide adequate adhesive properties cannot befixed at both ends of each tube with a resin adhesive without collapsingthe portions to be bonded during handling, or letting the adhesivepenetrate through the walls of tubes to block the interior.

As mentioned above, a thin porous film material made of atetrafluoroethylene resin can be provided with an increased flexibilityand higher degree of bendability by drawing the material at asufficiently high draw ratio to provide a higher porosity, but at thesame time, a tube made of such material tends to easily collapse into aflat form. Methods for avoiding this problem are described in U.S. Pat.Nos. 4,304,010 and 4,306,318, a tubular thin porous film material isdrawn at a given ratio and fired, and after inserting a cylindricalmetal member through the tube, the latter is heated in a radiant furnaceuntil the outer surface of the tube becomes over-sintered. However, thisis not industrially advantageous, since it involves great difficulty intreating the surface of an elongated article in a continuous fashion.

SUMMARY OF THE INVENTION

One object, therefore, of the present invention is to overcome theproblems described above, and according to the present invention, partof the surface of a thin porous film material made of atetrafluoroethylene resin is removed by heating said surface to atemperature higher than the thermal decomposition point of thetetrafluoroethylene resin. By this method, the thin porous film materialis provided with an adhesive surface while permitting the film materialto retain the desired pore size, hardness, and degree of penetration ofan adhesive.

Another object of the present invention is to improve the bendability ofa tubular thin porous film material. In accordance with the presentinvention, continuous surface treatment of an elongated article can berealized while ensuring high bendability without causing any practicalproblems.

A further object of the present invention concerns a tubular thin filmmaterial with asymmetric pore diameters wherein the fibrous structure ofa tetrafluoroethylene resin of which said material is made continuouslyvaries through said tubular thin porous film material so as to providethe outer and inner surface of the tube with different structures. Thepresent invention enables the pore sizes in said material to be freelycontrolled by heating its surface to a temperature higher than thethermal decomposition point of the tetrafluoroethylene resin so as todecompose and remove part of the surface by a desired thickness.Further, in accordance with the present invention, the pore size in thetubular thin porous film material can be varied with high precision inthe longitudinal direction of the tube.

The present inventors have made extensive studies on techniques fortreating the surface of a thin porous film material that has beenprepared by rolling or paste-extruding tetrafluoroethylene resin with aliquid lubricant into a thin film that is subsequently dried, drawn, andfired. As a result, the inventors have found that a thin porous filmmaterial having improved adhesive and bending properties can be obtainedby heating the surface of the resin to a temperature higher than itsthermal decomposition temperature so as to decompose and remove part ofsaid surface. The inventors have also found that this method enablescontrol of pore sizes in the material. The present invention has beenaccomplished on the basis of these findings.

DETAILED DESCRIPTION OF THE INVENTION

A thin porous film material made of a tetrafluoroethylene resin havinghigh resistance to heat and chemicals, good electrical insulatingproperties, high water repellency and good biocompatibility can beprovided with improved adhesive properties by treating its surface inaccordance with the method of the present invention. The thus-treatedmaterial can be easily assembled into an apparatus that is capable ofprecise and large-volume filtration, concentration or separatingoperations on an industrial scale. The material is therefore useful inindustrial fields where no economic advantages have been found in usingthe prior thin porous film materials because of the need for bulkprocessing.

The material treated by the method of the present invention thatexhibits the features of good biocompatibility, good adhesiveproperties, high flexibility and structural anisotropy can also be usedadvantageously in medical fields by incorporation in membranes forseparating blood corpuscles, artificial organs (e.g., artificial lung,kidney, and blood vessels), and medical equipment (e.g., catheters).Furthermore, the material treated in accordance with the presentinvention can be used for office equipments advantageously such as anoil feeder of a copy machine which is capable of feeding silicone oilquantitatively and preventing the adhesion of toner.

The method of the present invention is applicable to thin porous filmmaterials of tetrafluoroethylene resin whether they are in a sheet, rodor tubular form.

When the surface of the thin porous film material of tetrafluoroethyleneresin is heated to a temperature higher than the thermal decompositionpoint of the resin, the material is decomposed to undergo evaporationfrom the surface toward the interior. The thinnest filaments in theresin are the first to be decomposed and evaporated, and the thicknerfilaments and nodes are decomposed at a later stage, thereby providingroughened areas in the outermost layer where the apparent pore sizes aregreater than those in the interior of the resin. The degree of surfaceroughening, reflected in the pore sizes in the outermost layer and thedepth of the roughened surface, can be adjusted by varying thetemperature or duration of heating. It is particularly preferred thatheating is conducted for a short period at a higher temperature becauseby shortening the heating period, the amount of heat transmitted to theinside of the thin porous film material is reduced and only part of itssurface can be roughened without causing any change in the interiorporous structure. As a result, the thin porous film material is providedwith an adhesive surface while retaining the desired pore size,hardness, and degree of penetration of the adhesive.

The effects of the present invention are characterized by that the onlythe surface of the thin film material of tetrafluoroethylene resin canbe treated that results in the anchorage effect. As the result, theimprovement in the adhesion property can be attained without thedeterioration of the porosities of the whole material such as the porediameters and the hardness. Although the oxidized layer may be formed onthe surface of the material by the treatment according to the presentinvention which can improve the adhesion property, the principalcontribution for the improvement in adhesion property is attained by theancorage effect.

In treating the tubular thin porous film material, since the thickerfilaments and nodes in the resin are decomposed and evaporated laterthan the fine filaments, not only is the bendability of the thin porousfilm material improved but also the strength of the material in theradial direction is maintained sufficiently to avoid the chance of thetubular form thereof being collapsed into a flat form.

If the thin porous film material to be treated is a tubular materialwith asymmetric pore diameters, the pore sizes in the material can becontrolled by adjusting the depth of the surface layer to be treated. Inthe tubular thin film material with asymmetric pore diameters, the outersurface contains shorter filaments while the inner surface compriseslonger filaments, thereby providing a smaller average pore size in thearea closer to the outer surface and a larger average size in the areacloser to the inner surface. Therefore, a smaller average pore size canbe obtained by treating the outer surface of the tube to a small depth,while a greater average pore size can be realized by treating to adeeper area.

The term "thermal decomposition point" of tetrafluoroethylene resin usedin the present invention means the temperature at which thetetrafluoroethylene resin being beated at an atmospheric pressureundergoes an appreciable loss in weight. The thermal decomposition pointdepends on period of time for which heat treatment is carried out. Inone sense, 260° C. may be considered as a thermal decompositiontemperature, since the weight of the resin gradually dereases if it isexposed to this temperature for a prolonged period. However, for thepurposes of the present invention, 260° C. should not be included in themeaning of the "thermal decomposition point" of the tetrafluoroethyleneresin.

Although there is observed no transition point for the decrease in resinweight generally an appreciable loss in the weight of the resin occursfor the first time at a temperature in the range of about from 400° to500° C. For the convenience's sake, the thermal decomposition point ofthe tetrafluoroethylene resin constituting the thin porous film materialto be treated by the present invention may be assumed to be within thistemperature range.

Any heating method may be employed and illustrative examples include theuse of radiation furnaces, the use of metal rolls or a salt bath, theuse of hot air, the use of a flame from a gas burner, and the use of alaser beam. Hot air, a flame, and a laser beam are advantageously usedbecause they are easy to control. In the first case, for example, hotair at from 600° to 700° C. may be supplied by feeding air into a beaterwith a fan. In the second case, where a flame is used, its temperaturemay be controlled by, for example, mixing a flammable gas, e.g., propanegas, etc., with air having an increased oxygen concentration. In thethird case, a CO₂ gas laser may be used with advantage.

The heating period will vary with the depth of the surface layer to betreated, the density of the area to be treated of the shaped porous thinfilm, the heating temperature, and the specific heating means used. Alonger heting period is required if a deep area of the surface layer, ora thick film must be treated, or if the area to be treated of the shapedthin porous film has high density. On the other hand, a shorer heatingperiod should be selected if the heating temperature is high or if theheating element used has a large specific heat. As a guide, the heatingperiod should preferably be limited to 30 seconds or less if air hotterthan 600° C. is used, and a period of 1 second or less is preferablyselected if a CO₂ gas laser or a flame is used.

The following examples are provided for further illustrating theadvantages of the present invention but should not be taken as limitingits scope.

EXAMPLE 1

A porous tetrafluoroethylene resin tube (outside diameter: 0.9 mm,inside diameter: 0.2 mm, porosity: 33%) was found to have a bubble pointof 1.54 kg/cm². The "bubble point" is the pressure at which air bubblesare evolved when the interior of the tube is pressurized with air inisopropyl alcohol, and this pressure is a measure of maximum pore size.The tube was found to be capable of bonding to silicone rubber at astrength of 1.5 k g/cm².

The porous tetrafluoroethylene resin tube was transported at a linearspeed of 5 m/min for treating its surface by exposure to the tip of theflame from a gas burner issuing a mixture of propane gas (32%), oxygen(54%) and air (14%).

The thus-treated tube experienced little change in the outside diameter,inside diameter, or porosity, but its bubble point was reduced to 0.65kg/cm², and the strength of adhesion to silicone rubber increased to 5.4kg/cm².

EXAMPLE 2

The procedures of Example 1 were repeated except that the tube wastransported at a linear speed of 10 m/min. during the heat treatment.The thus-treated tube experienced little change in the outside diameter,inside diameter and porosity, but its bubble point was reduced to 0.92kg/cm² and the strength of adhesion to silicone rubber increased to 2.8kg/cm².

COMPARATIVE EXAMPLE 1

Two porous tetrafluoroethylene resin tubes respectively having bubblepoints of 0.92 kg/cm² and 0.65 kg/cm² were found to adhere to siliconerubber at strengths of 1.5 kg/cm² and 2.3 kg/cm², respectively, whenthey were not subjected to heat treatment in accordance with the presentinvention.

EXAMPLE 3

A porous tetrafluoroethylene resin tube (outside diameter: 1.1 mm,inside diameter: 0.3 mm, porosity: 35%) was found to adhere to siliconerubber at a strength of 1.8 kg/cm². This tube was transported at alinear speed of 5 m/min as it was irradiated with CO₂ gas laser beam ata power density of 5.75 W/mm². The thus-treated tube had an adhesivestrength to silicone rubber of 5.6 kg/cm².

EXAMPLE 4

A tetrafluoroethylene resin tube with asymmetric pore diameters (outsidediameter: 1.1 mm, inside diameter: 0.45 mm, porosity: 31%, bubble point:2.5 kg/cm²) was internally pressurized with water, but no water leakageoccurred even at 6 kg/cm². After treating part of the tube surface withhot air (ca. 700° C.) for 6 seconds, the tube was subjected to internalhydraulic water pressure. At a pressure of about 3 kg/cm², tiny drops ofwater gradually leaked out of the treated area of the tube. Aftercutting off the treated area, another portion of the tube was treatedwith hot air (ca. 700° C.) for 15 seconds and hydraulic pressure wasinternally exerted on the tube. At a pressure of about 1 kg/cm², waterdrops began to leak out of the treated portion.

A thin porous film material made of a tetrafluoroethylene resin havinghigh resistance to heat and chemicals, good electrical insulatingproperties and high biocompatibility can be provided with improvedadhesive properties by treating its surface in accordance with themethod of the present invention. Because of such improved adhesiveproperties, the thus-treated material can be easily assembled into anapparatus that is capable of large-volume processing and which will findexpanded utility in industry. The material treated by the method of thepresent invention, exhibiting the features of not only highbiocompatibility, but also good adhesive properties, high flexibility,and structural anisotropy, can also be used advantageously in medicalfields by use for plasma separation membranes, artificial organs such asartificial kidney, artificial lung, and artificial blood vessels, andmedical equipment such as catheters. Furthermore, the material treatedin accordance with the present invention can be used for officeequipment advantageously such as an oil feeder of a copy machine whichis capable of feeding silicone oil quantitatively and preventing theadhesion of toner.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein without,departing from the spirit and scope thereof.

What is claimed is:
 1. A thin porous film material oftetrafluoroethylene resin having enhanced adhesive properties, which hasbeen treated by heating the surface thereof to a temperature of about400° C. or more in a controlled manner effective to decompose and removea desired part of said surface, thus providing the thin porous filmmaterial with an adhesive surface without adversely affecting otherproperties of the thin porous film material.